These blood cells transport oxygen around all parts of the body using the blood system.

Haemoglobin is a type of protein. It is made up of four different parts. Each of these parts has a haem ring with a Fe atom in its centre. This is chemically attached to the protein by four strong bonds.

Note: a ‘chemical bond’ means that the two atoms involved share electrons.

Iron is capable of forming 6 bonds. In haemoglobin it sits within a haem ring, binding to four amino acids. A molecule of oxygen will attach at one of the remaining empty sites.

Haemoglobin: A ribbon representation of the protein. The 4 green areas show the Fe haem centres where oxygen will bind. Diagram: Wikipedia Commons, first uploaded by Zephyris and adapted by Richard Wheeler

An intricate balancing system is required so that oxygen will bind to Fe in haemoglobin (to be transported) but when cells around the blood system need it, the oxygen will be released.

Basically, the strength of the Fe-O bond has to change based on how much oxygen is around. Scientists have tried to understand how this works by using a ‘spring-tension’ model.

The reason that inhaling CO (carbon monoxide) is fatal, is because CO will bind to haemoglobin molecules. This prevents the transport of oxygen to cells which in turn prevents cell respiration and effectively starves cells to death.

The Spring-Tension Model

This model says that a haemoglobin molecule changes its shape depending on the concentration of oxygen around it.

It’s sort of like a crab claw.

Diagram of the Spring Tension Model, used to understand haemoglobin and oxygen binding in high and low oxygen concentrations. Source: Lecture notes from University College Dublin course: ‘Metals in Biology’

In the lungs and other areas where there is lots of oxygen, each Fe centre in the haemoglobin forms a strong bond to an oxygen molecule. This can be compared to a crab closing its pincher around a piece of food.

The formation of a Fe-O bond (an oxygen molecule attaches to the haemoglobin molecule) causes the protein to change shape, becoming more compact. This signals the other three Fe centres in the protein to bind oxygen quickly.

This is like signalling the other pincher of the crab to close, getting a better grip on the crab food.

In areas where there is not much oxygen, the presence of carbon dioxide gas signals the haemoglobin to release oxygen. The Fe-O chemical bonds between the haemoglobin iron centre and the oxygen become much weaker.

Once one Fe-O bond has broken, the protein gets bigger and this triggers the other Fe centres again to release their oxygen.

In our crab analogy, the crab looks at what is in its pinchers and decides it would prefer to eat something else. The crab releases its pinchers, one at a time.